1. Field of the Invention
The present invention relates to an electromagnetic induction casting apparatus which is used, for example, to manufacture a silicon ingot for a solar cell.
2. Description of the Prior Art
As one of methods for manufacturing unidirectionally solidified ingots of silicon used in solar cells, an electromagnetic induction casting method disclosed in Japanese Patent Laid-Open Nos. 2-30698, 4-338195 are known, for example. A typical electromagnetic induction casting method uses an induction coil 3, an electrically conductive bottomless crucible 2 which is disposed inside the induction coil 3 and a heat insulating furnace 4 which is disposed under the bottomless crucible 2 as shown in FIG. 8.
The electrically conductive bottomless crucible 2 has a structure in which at least a vertical portion is divided by vertical slits 2xe2x80x2, 2xe2x80x2, . . . into a plurality of vertically elongated sections 2xe2x80x3, 2xe2x80x3, . . . in a circumferential direction, cooperates with the induction coil 3 to electromagnetically melt a raw material supplied into the above described crucible and then allow the melt of raw material to be solidified. In order to solidify the melt of raw material and protect the crucible, the bottomless crucible 2 is configured to have a water-cooled structure which allows cooling water to pass through the bottomless crucible 2.
The heat insulating furnace 4 controls a temperature gradient by heating a solidified ingot which is pulled downward from the bottomless crucible with an electric heater 5.
Since the electrically conductive bottomless crucible 2 is divided by the vertical slits 2xe2x80x2, 2xe2x80x2, . . . into the plurality of vertically elongated sections 2xe2x80x3, 2xe2x80x3, . . . , the electromagnetic induction casting method not only heats and melts the raw material in the crucible by electromagnetic induction but also produces a repulsive force between the crucible and the raw material in the crucible, thereby lessening contact between the crucible and the raw material. When contact is lessened as described above, the raw material is less contaminated, a product quality is improved, the mold is not consumed substantially and an equipment cost is lowered. Furthermore, a casting efficiency is improved by continuous casting. Accordingly, a high quality silicon ingot is manufactured economically.
In relation to the electrically conductive crucible 2 which has the water-cooled structure, however, this electromagnetic induction casting method poses problems which are described below.
The inventors have long been making researches to manufacture a high quality silicon ingot for a solar cell by the electromagnetic induction casting method. In the course of these researches, the inventors found that the performance of silicon ingot as a solar cell was improved with a leap by controlling a temperature gradient of silicon within a range of 15 to 25xc2x0 C. in a relatively narrow temperature range from 1420xc2x0 C. which is a melting point of silicon to 1200xc2x0 C. and disclosed this knowledge by Japanese Patent Laid-Open No. 4-342496.
Reasons why the control of the temperature gradient is effective for the performance improvement consist in a fact that a large number of defects which lower a photoelectric conversion efficiency of a solar cell are produced while silicon passes through the temperature range from 1420xc2x0 C. to 1200xc2x0 C., a fact that thermal stresses produced in crystals are moderated and production of crystalline defects is prevented by lowering a temperature gradient in this temperature range and the like.
A conventional electromagnetic induction casting apparatus has a heat insulating furnace 4 which is disposed under a bottomless crucible 2 and is capable of controlling a temperature gradient of an ingot after the ingot is pulled down under the bottomless crucible 2. By the way, an ingot temperature at an upper end of the heat insulating furnace is 1300 to 1000xc2x0 C. However, an inside surface of the bottomless crucible 2 is cooled with water to 200xc2x0 C. or lower. Accordingly, silicon in the bottomless crucible is rapidly cooled with the inside surface of the crucible which is forcibly cooled with water, whereby too large temperature gradient can hardly be restricted even with the heat insulating furnace 4 within a range of 15 to 25xc2x0 C./cm in the temperature range from 1420xc2x0 C. to 1200xc2x0 C. which produces a large influence on the performance of the silicon ingot as a solar cell.
In addition to the rapid cooling of silicon in the bottomless crucible, the temperature gradient is partially improved due to an rapid temperature change from a low temperature zone (200xc2x0 C. or lower) of the inside surface of the crucible to the upper end of the heat insulating furnace (on the order of 1300 to 1000xc2x0 C.). Silicon which is a fragile material is apt to be cracked due to the rapid cooling and the partial improvement of the temperature gradient.
An object of the present invention is to provide an electromagnetic induction casting apparatus which is capable of restricting a temperature gradient of an ingot within a narrow range immediately after solidification in an electrically conductive bottomless crucible.
In order to accomplish the above described object, an electromagnetic induction casting apparatus according to the present invention is an electromagnetic induction casting apparatus configured to electromagnetically melt a raw material in an electrically conductive bottomless crucible which is disposed inside an induction coil and has at least a vertical portion divided by vertical slits into a plurality of portions in a circumferential direction, and pull down the melt of raw material downward while allowing the raw material to be solidified, wherein an upper section and a lower section of the above described electrically conductive bottomless crucible are configured as a water-cooled section and a non-water-cooled section respectively, and at least a vertical portion of the water-cooled section and at least a vertical portion of the non-water-cooled section are divided by vertical slits into a plurality of portions in the circumferential direction.
The upper section of the bottomless crucible must have a cooling capability sufficient for starting solidification of melt. For this reason, it is necessary to cool the upper section of the bottomless crucible with water and it is preferable for obtaining the cooling capability to make the upper section of the bottomless crucible a metal having a high heat conductivity such as copper, silver or the like.
For the lower section of the bottomless crucible which faces a skin of a solidified ingot, on the other hand, a water-cooling structure will cool the lower section excessively, thereby causing excessive cooling of the ingot immediately after solidification. Accordingly, a non-water-cooling structure is adopted for the lower section of the bottomless crucible. Since the non-water-cooled lower section is an electrically conductive section and has the slits like the upper water-cooled section, even the non-water-cooled lower section is capable of positively heating inside of the crucible by electromagnetic induction like the upper section of the crucible. In addition, the non-water-cooled section is heated and the ingot can be heated by radiation from the non-water-cooled section.
Owing to the items described above, the electromagnetic induction casting apparatus according to the present invention is capable of preventing an ingot from being cooled excessively in the lower section of the bottomless crucible and restricting a temperature gradient of the ingot immediately after solidification. Furthermore, the electromagnetic induction casting apparatus according to the present invention moderates an rapid temperature change from the bottomless crucible to the heat insulating furnace, thereby preventing the temperature gradient from being partially improved due to the temperature change.
It is preferable that the non-water-cooled section has a slit length at a ratio of 10 to 50% of a total slit length. At a ratio of lower than 10%, it will be difficult to carry out induction heating effectively inside the lower section of the crucible. At a ratio of higher than 50% on the other hand, a solidification start line (a triple point of the melt, the crucible and a solidified portion) will be brought into contact with the lower section of the crucible and a cooling effect sufficient for starting solidification will not be obtained, thereby making it difficult to solidify the melt stably and resulting in a possibility of the leakage of the melt.
A total height of the bottomless crucible, a height of the water-cooled section and a height of the non-water-cooled section are set so that the above described slit length can be obtained. For reference, a total slit height is on the order of 400 to 500 mm.
It is preferable to form slits continuously from the water-cooled section to the non-water-cooled section by connecting the slits in the water-cooled section to the slits in the non-water-cooled section. When the slits are formed as described above, an electromagnetic force is not discontinuous even in the vicinity of a boundary between the water-cooled section and the non-water cooled section. Accordingly, casting is more stabilized and an energy efficiency is improved.
As a material for the non-water-cooled section, it is preferable to select an electrically conductive material having a high melting point such as molybdenum, tungsten, titanium or the like. Such a material poses no problem even when the non-water-cooled section is partially heated to a high temperature exceeding 1000xc2x0 C.
The induction coil outside the crucible can be disposed independently outside the water-cooled section and the non-water-cooled section of the bottomless crucible, respectively. When induction coils are disposed independently as described above, heating of an interior of the non-water-cooled section is accelerated and a control accuracy of a heating temperature is improved. Accordingly, a temperature gradient is restricted more effectively.
The bottomless crucible may be of an assembling type which can be separated into the water-cooled section and the non-water-cooled section. The assembling type crucible can easily be restored when the crucible is deformed. Furthermore, the assembling type crucible permits partial exchange of the crucible and lowers a cost required for exchange.
The electromagnetic induction casting apparatus according to the present invention is suited to manufacturing of a silicon ingot, poly-silicon ingot for a solar cell in particular, and is applicable also to manufacturing of other semiconductors and metals.